The overall aim of this proposal is to develop specific inhibitors of Inosine Monophosphate Dehydrogenase (IMPDH) for eventual clinical application as immunosuppressants. IMPDH requires a cofactor, nicotinamide adenine dinucleotide (NAD), to carry out its biological function. We synthesized analogues of NAD, which have been designed to bind the cofactor domain of IMPDH but are incapable of carrying out the coenzyme function resulting in inhibition of the enzyme. The specific affinity towards IMPDH has been achieved by replacement of the nicotinamide riboside portion of NAD with mycophenolic acid (MPA) derivatives. MPA is a specific inhibitor of human IMPDH type II, an isoform over-expressed in activated lymphocytes. It binds at the coenzyme pocket of IMPDH mimicking the binding of nicotinamide riboside; the adenosine binding sub-site of IMPDH remains empty. Therefore, we linked adenosine derivatives with mycophenolic acid analogues through methylenebis(phosphonate) linkage to give mycophenolic adenine dinucleotide (MAD) analogues. MADs were found to be as potent suppressors of alloantigen-induced proliferation of human lymphocytes as MPA. Although, MPA has been approved by the FDA, its clinical potential is limited by its undesirable rapid glucuronidation. In humans as much as 90% of the drug is converted into the inactive glucuronide. In contrast to MPA, our MAD analogues were resistant to glucuronidation. They were, however, a little less potent as inhibitors of IMPDH. Therefore, we propose herein the synthesis of three difluoromethylenebis(phosphonate) analogues of MAD. As isosteric and isopolar to natural pyrophosphates, these analogues are expected to be more potent than the corresponding methylenebis(phosphonate)s. In addition, the most potent CF2-MAD analogues will be substituted at the C2 of the adenine moiety with amino and oxo function. This is expected to improve the specificity of such inhibitors against isoform type II of IMPDH. Indeed, the crystal structure of IMPDH type II (recently solved by us) and our preliminary modeling studies, showed that MADs containing 2,6- diaminopurine or isoguanine should fit well to the isoform II (expressed in activated lymphocytes). They would not be accommodated by isoform type I (abundant in normal cells) due to steric hindrance. New MAD analogues will be tested (in comparison to MPA) as inhibitors of IMPDH type I and type II and inhibitors of proliferation of human lymphocytes. The toxicity of these analogues will be evaluated in an animal model. PROPOSED COMMERCIAL APPLICATIONS: IMPDH inhibitors are of commercial interest to several major drug companies. Among them, only mycophenolic mofetil has been developed as immunosuppressant. However, its clinical use is limited due to undesirable metabolism. Certainly, new and specific inhibitors of IMPDH are expected to be of great therapeutic interest and commercial potential.